Rubber compositions for vehicles tires frequently use reinforcing fillers, such as carbon black, to impart desirable properties such as abrasion resistance and rolling resistance. The rubber molecules and carbon black particles interact through cross-linking. The amount of cross-linking must be controlled, however. Insufficient cross-linking can lead to the rubber having poor properties (e.g., abrasion resistance, hardness, etc.) and too much cross-linking can lead to the rubber composition being brittle and hard to process.